The invention relates to a method of separating impurities in connection with the chemical recovery system of a pulp mill, in particular from the calcium-containing substances used therein, which are mainly lime sludge and lime, and from its liquor circulation.
The term impurities is used in the present patent application to denote various detrimental substances such as silicon, phosphor, vanadium, sulphur, etc., which may also be called non-process elements, building up in the chemical circulation of a pulp manufacture process and originating from various raw material sources. Sources of impurities are mainly wood raw material (phosphor), make-up lime (silicon) and combustion oil (vanadium, sulphur) if oil is used as fuel in the lime sludge reburning kiln. Some raw materials used in the manufacture of cellulose pulp also may contain in their cells lots of silica (SiO.sub.2). The most usual of these are annual plants such as bamboo, sugar cane, rice and wheat. It has also been discovered that some tropical wood species contain such amounts of silicon that are detrimental for pulp production processes.
When pulp is manufactured using the sulphate method the cooking liquor used, or black liquor, is evaporated, combusted and the residue obtained, the so-called soda smelt, is dissolved in water. The green liquor thus formed is causticized to produce cooking liquor. During the causticizing process, the sodium carbonate contained in the green liquor is allowed to react with burnt lime (CaO) as follows. First, lime slaking takes place: EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 (1)
Subsequently, the causticizing reaction itself takes place: EQU Ca(OH).sub.2 +Na.sub.2 CO.sub.3 .fwdarw.2NaOH+CaCO.sub.3 (2)
The mixture thus produced contains sodium hydroxide (white liquor) and calcium carbonate (lime sludge) is separated and the white liquor is recirculated to pulp digestion. White liquor and lime sludge may be separated either by settling or filtering. White liquor filters are usually conventional candle filters or drum or disc filters operating with pressure or vacuum. Lime sludge is washed in order to remove alkali therefrom. Lime sludge is thickened in a lime sludge filter for combustion in a calcinating device such as lime sludge reburning kiln in which the lime sludge is regenerated to calcium oxide to be returned to the causticizing. This circulation process of lime is called the lime cycle.
In the chemical recovery system of a pulp mill the material flow described above is partly cleaned as part of the soda smelt does not dissolve in the soda dissolving stage and the so-called green liquor sludge formed is removed. Undesirable substances may thus be removed from the process in this way.
The problem in the process is that certain impurities are not easily removed because they are not sparingly soluble enough and thus to a large extent remain in the solution. For example phosphor, silicon and vanadium compounds as well as many other compound are this kind of substances. These compounds tend to precipitate during causticizing in the lime sludge and thus build up in the lime cycle. For example silicates precipitate as calcium hydrosilicates to the lime used in the causticizing process. In this way, impurities are accumulated with time in the lime sludge which means that the lime sludge is contaminated. Phosphor tends to concentrate in the fine dust of the lime sludge reburning kiln and it has been desirable to try to remove it in this form. Fine dust has sometimes been used as phosphor-containing soil conditioner.
Contamination of lime sludge results in various problems, for example deposit rings and balls accumulate in the lime sludge reburning kiln, filtration of white liquor becomes difficult and burnt lime passivates, i.e. it is bound in non-reactive compounds. For this reason, at least a part of the lime sludge must be replaced from time to time, i.e. a part of the lime sludge must be removed continuously and the lime cycle must thus be kept open to some extent.
A further problem is that the lime sludge discharged must either be transported to a landfill area or used to neutralize acid effluents from a pulp bleaching process whereby phosphor and other impurities end up in watercourses.
Further, as the tendency today is to close the water circulation of a pulp mill and to return the bleaching effluents to the process the impurity load of the chemical circulation system increases and drawbacks become more severe.
Instead of destroying the fouled lime it has been suggested to separate the silica already from the black liquor by lowering the pH of the liquor with carbon dioxide to a range of about 9.1-10.2. The solubility of the silica dissolved in the black liquor in ion form, mostly as HSiO.sub.4.sup.3- and SiO.sub.3.sup.3-, decreases and it precipitates as colloidal silica gel. It has been reported that even 90% of the silicate may be separated from weak black liquor (6 g SiO.sub.2 /l) by a method developed by The United Nations Development Organization (UNIDO) and the Swedish International Development Authority (SIDA). In that case carbon dioxide is bubbled in a bubbling reactor into black liquor and subsequently the precipitated silica is separated by filtering. CPPRI and Lurgi have developed similar approaches.
Silica may be separated from green liquor with carbon dioxide in the same way as from black liquor. Green liquor may contain about 10-20 g SiO.sub.2 /l of silicate, sometimes even more, depending on the raw material used, which is remarkably more that in weak black liquor, and therefore better yield may be expected. Separation of silica precipitate from green liquor is not as problematic as from black liquor since organic matter is not present in green liquor. A drawback of this method is that odorous sulphur-containing compounds such as hydrogen sulphide are released when sulphate liquor is treated with carbon dioxide.
Another known method of removing silicon from green liquor is to add lime (CaO or Ca(OH).sub.2) to the green liquor whereby silicon precipitates with the lime and this lime rich in silicon may be removed. It has been suggested to divide the causticizing process of green liquor in two stages and to add part of the total volume of lime required in the causticizing process to the first stage. Then silicon precipitates as calcium silicate in the lime sludge produced and the silicon-rich lime sludge may be discharged. The remaining portion of the lime is added to the second stage and this lime is circulated in the process. A limitation of this method is that a high silicon removal rate is achieved only if the silicon content of the green liquor is high, for example more than 12 g/kg H.sub.2 O.
The object of the present invention is to eliminate the drawbacks mentioned above.
In particular the object of the invention is to provide a new kind of method of separating detrimental impurities from the chemical circulation of a pulp mill, particularly from lime cycle.
A further object of the invention is to prolong the operating life of lime sludge in a pulp process.
A further object of the invention is to provide a method of recovering more efficiently and reusing the useful chemicals of a pulp process. Particular attention is paid to a feature of separating the impurities discussed above in a readily separable form and from solutions which contain low concentrations of these impurities.
Further, an object of the invention is to provide a method by which the chemical circulation of a pulp process may be closed as far as possible.
The characteristic feature of the invention are defined in the appended patent claims.
The invention is based on the observation made in tests performed that by soaking lime-containing material such as lime sludge in a solution of carbonate or hydrocarbonate, preferably having a high concentration of carbonate or hydrocarbonate, anions (CO.sub.3.sup.2-, HCO.sub.3.sup.-) which are difficult to separate are dissolved and may thus be separated from the lime sludge or lime. Thus, the invention makes use of the solubility properties and products in the conditions mentioned in a new way.
In the context of the present invention the term lime means material containing calcium oxide (CaO) or calcium hydroxide (Ca(OH).sub.2) used in the mill for carrying out for example causticizing processes. The term lime sludge means material which contains calcium carbonate (CaCO.sub.3)
The solubility of sparingly soluble substances is often expressed as solubility product as follows: EQU L.sub.MemXn =[Me].sup.m [X].sup.n (3)
in which Me is a metal ion; X is an anion forming a sparingly soluble salt with the metal ion in question; m and n are integral numbers; parentheses [ ] express concentration; and L is solubility product which with certain conditions is constant.
The solubility product is in fact constant only with dilute solutions in which the ionic strength or the value expressing the summed concentration of all ions is low. If there are large amounts of dissolved salts in the solution which means that the ionic strength is high the equation (3) is valid only if the concentrations are replaced by activities. The connection between the activity and the concentration is: EQU a=F[A] (4)
in which a is the activity of the substance; [A] is its concentration and f is the activity coefficient. The solubility product is then: EQU L.sub.MemXn =a.sub.Me.sup.m a.sub.x.sup.n (5)
Information on the activity coefficients are often not available for concentrated solutions having complicated compositions. However, it is not necessary to know them in order to recognise the efficiency of the method of the invention.
If anion X is a carbonate, rising its activity results in a decrease in the activity of all metal ions forming sparingly soluble carbonates while the solubility product remains constant. This presupposes that their concentration in the liquid is sufficiently high to be in balance with solid carbonate.
Me may be any metal ion meeting these requirements. The carbonate activity may be raised by a readily soluble salt such as sodium carbonate. A decrease in the activity of the metal ions results, according to equation (5), in dissolving of other anions X than carbonate, which also form sparingly soluble compounds with the same metals and are present in the form of precipitate in the lime sludge; i.e. the activities of anions in the solution increase.
Metals which form poorly soluble carbonates in the conditions discussed and the activity of which in the solution may be decreased, are primarily calcium, barium, iron, manganese, magnesium, etc. Anions the activity of which may be raised by this method are primarily PO.sub.4.sup.3-, SiO.sub.3.sup.2-, VO.sub.4.sup.3-, SO.sub.4.sup.2-, etc.
The exact form of the anion, for example SO.sub.4.sup.2- vs. SO.sub.3.sup.2- or VO.sub.4.sup.2- vs. V.sub.2 O.sub.5.sup.2-, need not necessarily be known, neither the metal with which it has precipitated. The only prerequisite of the method of the invention is that anions and cations are in balance with the solid phase according to equation (5).
Sparingly soluble compounds the anions of which may be dissolved when the activity of the metal ion decreases are for example the following: EQU L.sub.Ca3 (PO4)2 =a.sub.Ca3 .multidot.a.sub.PO42 calcium phosphate EQU L.sub.CaSiO3 =a.sub.Ca .multidot.a.sub.Sio3 calcium metasilicate EQU L.sub.CaSO4 =a.sub.Ca .multidot.a.sub.SO4 calcium sulphate EQU L.sub.Ca3 (VO4)2 =a.sub.Ca3 .multidot.a.sub.VO42 calcium vanadate EQU L.sub.CaHVO4 =a.sub.Ca3 .multidot.a.sub.HVO4 calcium hydrogen vanadate EQU L.sub.Fe3(PO4)2 =a.sub.Fe3 .multidot.a.sub.PO42 iron phosphate EQU L.sub.BaSO4 =a.sub.Ba .multidot.a.sub.SO4 barium sulphate
According to the method of the invention, lime sludge is soaked preferably in a strong solution of carbonate or hydrocarbonate, for example in a solution of potassium, sodium or ammonium carbonate or hydrocarbonate. An increased carbonate or hydrocarbonate content decreases the calcium concentration of the solution in a way required by the solubility product. The same happens with other cations forming sparingly soluble carbonates or hydrocarbonates. When the concentrations of calcium and other cations decrease the solubility of anions, such as phosphate, silicate, vanadate, sulphate, sulphite, etc., which form sparingly soluble salts with them, increase correspondingly in a way required by the solubility product of the salts. These anions precipitated in the lime sludge are thus dissolved. After the soaking, the lime sludge and the solution, i.e. the dissolved impurities are separated from each other by any method known per se.
When for example a solution of sodium carbonate is used as the lime sludge soaking solution, the sodium is purified and reused or it is recycled to cooking chemical preparation. Separating the sodium carbonate by crystallization is advantageous as the sodium carbonate concentration remaining in the impurity concentrate is then as small as possible (see Table 1. Solubility of sodium carbonate in water).
The volume of the impurities concentrate to be removed may be adjusted by choosing for the soaking a suitable carbonate concentration, a suitable crystallizing temperature and by proceeding the crystallization suitably far. A suitable carbonate concentration when using for example sodium carbonate is about 5 g/l--saturated solution, preferably about 200-400 g/l. Sodium carbonate containing 7 to 10 crystal water, binds a large volume of water and the impurities may be concentrated in a small volume of liquid. The limit is set by the solubility of the impurities. If the limit is exceeded the impurities precipitate with the sodium carbonate and the purifying efficiency of the crystallization decreases. If necessary the carbonates may be dissolved in water and recrystallized, thus reducing the amount of impurities entrained with the carbonate back to the soaking.
TABLE 1 __________________________________________________________________________ Na.sub.2 CO.sub.3 / steam g mol/ 100 (g/g) pressure T.degree. solid 1000 g H.sub.2 O H.sub.2 O Sat. Sol density Hg __________________________________________________________________________ -2.10 Na.sub.2 CO.sub.3.10H.sub.2 O + ice 0.575 6.10 5.75 1.056 -- 0 " .66 7.0 6.54 -- -- 5 " .84 8.90 8.2 -- -- 10 " 1.14 12.1 10.8 -- -- 15 " 1.55 16.4 14.1 1.1515 12.3 20 " 2.09 22.2 18.1 1.1941 16.9 25 " 2.77 29.4 22.7 1.2416 21.4 30 " 3.70 39.2 28.2 1.342 26.8 32.00 Na.sub.2 CO.sub.3.10H.sub.2 O + 4.28 45.4 31.2 -- 29.0 Na.sub.2 CO.sub.3.7H.sub.2 O .sup.m 32.96 Na.sub.2 CO.sub.3.10H.sub.2 O + 4.71 49.9 33.3 -- 29.5 Na.sub.2 CO.sub.3.H.sub.2 O .sup.m 30 Na.sub.2 CO.sub.3.H.sub.2 O 4.78 50.7 33.6 -- 35.37 Na.sub.2 CO.sub.3.7H.sub.2 O + 4.67 49.5 33.1 34.0 Na.sub.2 CO.sub.3.H.sub.2 O 40 Na.sub.2 CO.sub.3.H.sub.2 O 4.60 48.4 32.8 43. 6 50 " 4.48 47.5 32.2 74.1 60 " 4.37 46.3 31.6 121.5 70 " 4.30 45.6 31.3 192.7 75 " 4.28 45.4 31.2 239.8 80 " 4.26 45.2 31.1 296.2 90 " 4.24 44.9 31.0 442.4 100 " 4.22 44.7 30.9 631.7 104.8 " 4.21 44.6 30.8 760.0 109 Na.sub.2 CO.sub.3.H.sub.2 O + Na.sub.2 CO.sub.3 4.20 44.5 30.8 1.15 (Atm) 110 Na.sub.2 CO.sup.3 4.20 44.5 30.8 1.19 " 113 " 4.20 44.5 30.8 -- " 120 " 4.03 42.7 29.9 1.65 " 130 " 3.86 40.9 29.0 2.25 " 140 " 3.71 39.3 28.2 3.02 " 150 " 3.57 37.8 27.4 4.01 " 160 " 3.44 36.5 26.7 5.27 " 180 " 3.16 33.5 25.1 8.67 " 200 " 2.89 30.6 23.4 13.7 " 220 Na.sub.2 CO.sup.3 2.56 27.1 21.3 21.0 " 240 " 2.16 22.9 18.6 30.9 " 250 " 1.95 20.7 17.1 37.0 " 260 " 1.75 18.6 15.7 44.2 " 280 " 1.32 14.0 12.3 61.7 " 300 " 0.88 9.3 8.5 83.8 " 350 " 0.19 2.0 2.0 166 " __________________________________________________________________________
The temperature at which the calcium-containing substance such as lime sludge is soaked affects the soaking efficiency. A high temperature is advantageous for the soaking efficiency but in view of the economy of the process it is favourable to use the waste heat from pulp digestion or other energy of little value, i.e. condensates of less than about 85.degree. C. Thus the more valuable primary energy in the form of steam need not be used. A suitable soaking temperature according to an embodiment of the method of the invention is 20.degree. C.--the boiling point of the solution preferably 80-110.degree. C. The boiling point depends on the concentration and pressure of the carbonate-containing solution used. If desired, the lime sludge may be soaked at normal pressure or at a desired superatmospheric or subatmospheric pressure.
The solution/lime sludge ratio (weight of solution/weight of lime sludge dry solids) in the lime sludge soaking may be of the order of 2-20, preferably 6-15.
According to the method of the invention the lime sludge and the carbonate solution, preferably sodium carbonate solution, used in the dissolving are separated from each other for example by filtering, centrifuging, sedimenting or by other known methods. The lime sludge may also be washed with water to intensify the cleaning.
The carbonate may advantageously be crystallized from the carbonate solution containing impurities. The carbonate and the solution containing impurities may preferably be separated from each other.
Crystallizing sodium carbonate at a temperature of 5-20.degree. C. produces pure decahydrate crystals and at a temperature of 35-90.degree. C. produces pure monohydrate crystals. The crystallizing may be practices for example by cooling crystallization which produces decahydrate crystals or by evaporation which produces monohydrate crystals. In conventional evaporation heat is brought to the process which evaporated water and thus raises the carbonate content of the solution over the limit required by crystallization. The carbonate solution may be evaporated in order to crystallize the carbonate in a way known per se by one-stage or multi-stage evaporator. The evaporation may be carried out in a tube, lamella or flash apparatus.
In an embodiment, the suitable crystallization temperature range for the carbonate is -2-30.degree. C. At the end stage a temperature of the solution of almost -2.degree. C. is required to accomplish as complete crystallization as possible. Outer air may be used for the cooling the temperature of which is low enough for the most part of the year at least in the Nordic countries.
If desired the carbonate may advantageously be dissolved in water; water, preferably lime sludge wash water, may be added to it and the liquid produced may be used in the soaking of the lime sludge.
A suitable temperature for the dissolving of the sodium carbonate crystals is for example about 50.degree. C. The condensate used for heating the soaking reactor may be used for raising the temperature to the desired level; in this way the heat may be reclaimed economically.
If desired the separated carbonate may be returned to the chemical circulation.
The solution containing impurities, the so-called concentrate may be discharged from the process. The impurities concentrate may be subjected to one or several recrystallization/s in order to recover useful chemicals, mostly carbonate crystals.
The above description refers mostly to the use of carbonate in the cleaning of lime sludge. However, also hydrocarbonate may be used in the solutions used for soaking the lime sludge in addition to the carbonate or to instead of it (0-100%). Hydrocarbonate may be recovered in a similar way as carbonate.
The water volume used in washing the lime sludge and the volume of the discharged solution containing impurities may preferably be balanced so as to keep the total volume of water in the process constant.
An interesting embodiment of the invention is removal of silicon and phosphor from lime sludge and lime containing high concentrations of these substances by dissolving them from the lime sludge or lime into a carbonate solution. A solution of this kind containing soluble carbonate forms with calcium sparingly soluble calcium compounds such as calcium carbonate and preserves the solubility of calcium low, thus no silicate precipitates. Preferably the solution of this kind is green liquor or some other solution containing carbonate, particularly potassium carbonate. If the carbonate solution such as green liquor contains also silicon the invention is preferably practised at a silicon concentration of about 0.8-12 g SiO.sub.2 /kg H.sub.2 O, preferably 1-6 g SiO.sub.2 /kg H.sub.2 O. When silicon has been removed from the lime sludge or lime to the solution the solution may be cleaned from these substances. For this purpose, such an amount of lime is added to liquor which is sufficient to precipitate the silicon from the liquor. Lime obtained by combusting lime sludge which has been purified earlier, or more reactive lime obtained for example by combusting porous limestone may be used for the precipitation. The advantages of the invention are obvious here as the volume of lime required for precipitating the silicon is substantially smaller than the volume of purified lime or lime sludge and thus the volume of silicon-rich lime sludge, which must be wasted, is small.
Dissoluble silicon reacts with lime and forms calcium hydrosilicates when the amount of silicon in the green liquor coming to the causticizing process is larger than in the white liquor produced. These hydrosilicates react in the lime sludge reburning kiln with lime and form calcium silicates in which the CaO/SiO.sub.2 ratio is higher than in the original calcium hydrosilicate. The calcium silicate most likely formed is .beta.-dicalcium silicate. The results of thorough thermodynamic analyses of the compounds formed by silicon and lime, experimental studies and computer simulations based on these show that in the causticizing the solution is not supersaturated with calcium silicate hydrate which is in metastable balance with .beta.-dicalcium silicate but the .beta.-dicalcium silicate coming in with the lime tends to dissolve from the lime to the solution until the amount of the dissolved silicon in the solution and the amount of added lime have reached a certain level. When the calcination advances the solution becomes supersaturated with calcium silicate hydrate and the silicon in the solution begins to precipitate as calcium silicate hydrate. Compounds of this kind are for example hydrosilicates like tobermorite (4CaSiO.sub.3 *Ca(OH).sub.2) and jennite (4CaSiO.sub.3 5Ca(OH).sub.2).
The invention is particularly advantageously applied in a causticizing process in a pulp mill. Most preferably the causticizing is practised in two stages and 40-90%, preferably 50-70% of the total lime volume required is added in the first stage in the flow direction of the green liquor (as generally known, the total lime volume required in the causticizing is the volume of lime, the amounts exceeding which cause calcium hydroxide to remain in the lime sludge to a harmful extent). When adding in the first stage the amount of lime mentioned silicon does not precipitate but on the contrary the silicon possibly contained in the lime dissolves from the lime to the liquor. Thus, from the first causticizing stage lime sludge is obtained which is clean as far as silicon is concerned, i.e. the silicon content of the lime sludge is substantially lower than that of the lime fed into the causticizing process. The rest of the lime required for the causticizing, i.e. 10-60%, preferably 30-50%, is added in the second stage and the silicon contained in the liquor is concentrated in the lime sludge. A part of this lime sludge may be discharged from the process in order to reduce the silicon load of the lime circulation and the rest may be mixed with the silicon-poor lime sludge obtained from the first stage. The lime sludge mixture is regenerated to lime in a lime sludge combustion apparatus such as lime sludge reburning kiln. The silicon and/or phosphor concentrations in the chemical circulation of a pulp mill may be controlled in this way.
Also the alkali content of the green liquor may have an effect when practising the invention. As known, green liquor is produced by dissolving the chemical smelt obtained from the combustion of black liquor in water or weak white liquor which is produced in washing of lime sludge after separation of white liquor. One of the main purposes of washing lime sludge is to remove alkali (NaOH) from the lime sludge as completely as possible. If separation of white liquor and lime sludge is not efficient, much alkali remains in the lime sludge and further in the weak white liquor. If weak liquor rich in alkali is used in the dissolving of smelt the alkali content of the green liquor will correspondingly be higher which in turn reduces the share of sodium carbonate in the green liquor. Pressurized disc filter, the use of which is presently becoming more common, separates more efficiently white liquor and lime sludge so that in the washing of lime sludge the content of NaOH in the weak white liquor is only about 5 g/l.
When the alkali content of the green liquor is kept as low as possible by dissolving the smelt in a solution containing as little NaOH as possible, the share of soluble sodium carbonate in the green liquor increases. Then the lime contained in the liquor reacts to a larger extent according to the reaction equation (2) and does not precipitate soluble silicon. Thus the amount of lime added in the causticizing stage may be increased and more silicon is dissolved in this causticizing stage and as a result also more cleaned lime sludge is obtained. Then the amount of lime in the final causticizing stage in which silicon accumulates is smaller.
The same kind of results are obtained when the silicon content of the green liquor is reduced before causticizing. This may be brought about, as previously described, by pretreatment of green liquor or black liquor with carbon dioxide-containing gas. An advantageous method of reducing the silicon content in green liquor is disclosed in PCT patent application no. PCT/FI95/00556 according to the method of which sodium carbonate-containing smelt obtained from combustion of black liquor is pretreated so that the sodium carbonate is recovered in solid form whereas silicon and/or phosphor are separated as a solution containing dissolved sodium silicates/sodium phosphates.
Causticizing may be practised also by a countercurrent method, in which liquor and lime flow in opposite directions. The process preferably comprises one step or several steps. In this case all the lime required in the causticizing is added in the last step relative to the liquor flow. Between the steps, lime/lime sludge and liquor are separated from each other. The liquor is transported to the next step and the separated lime/lime sludge is guided to the previous step relative to the liquor flow. The most impure lime sludge is the one separated from the last step, and a small portion (for example 1/5) of it is discharged from the process and the rest of it is guided countercurrent to the liquor flow and discharged from the first step.
The invention described above may be applied also elsewhere in a pulp mill and not only in the causticizing. Prior to concentration of the lime sludge, a part of the lime sludge flow may be treated with green liquor or a corresponding solution for example in a mixing tank to dissolve silicon. After this the lime sludge is separated, washed and transported to a lime sludge filter for thickening.
According to another embodiment lime sludge is treated for example outside the mill building in a similar way as low-grade ores are dressed with the so-called heap leaching method. In this case the leaching medium is green liquor or other carbonate solution with which the lime sludge heap is treated to remove silicon and corresponding impurities therefrom. This embodiment may be employable in some circumstances, for example in the treatment of old silicon-rich lime sludge supplies and subsequently the purified lime sludge of this kind may be reused. Silicon is removed from lime sludge treatment solutions by precipitating with lime whereby the solution may be circulated but silicon has been concentrated from the original to a remarkably smaller lime sludge volume which is discharged.
The life of lime sludge may be prolonged substantially by employing the invention, i.e. by cleaning the lime sludge or other calcium-containing material according to the invention. Further, the invention provides cost savings due to the longer life of lime sludge and saving of other chemicals; the demand of purchase lime drops and the landfill costs and costs for transport to a landfill area are reduced. By employing the invention, for example silicon may be removed from liquors which contain harmful amounts of silicon but, however, less than could be removed economically by known methods.
Further, due to the invention, less detrimental compounds from pulp production end up in the environment.
Further, the invention reduces the need of opening the lime circulation, i.e. it facilitates closing the chemical circulation.